Statistical analysis of the bathymetric distribution
Based on robust molecular data to distinguish between the shallow and deep species from other parts of the North Atlantic Ocean (Figure 4a, b), the historical specimens from Swedish Gullmar Fjord most probably belong to the deep-water species D. velifer and not to the shallow water D. robustus. This is because most of the specimens from the Gullmar Fjord were found in depths of 100–118 m, and never shallower than 53 m. This agrees with the general upper bathymetrical limit for D. velifer at 40–60 m. At some localities in the Artic seas where the layer of cold water can occur close to the surface, e.g. in the Kara Sea and the White Sea, D. velifer can be found in shallow depths, around 15–20 m, thus overlapping with the potential range of D. robustus. However, it is important to note that the true D. robustus was never found in Kara and White seas, except for the subarctic Barents Sea, which is influenced by the Gulfstream. Furthermore, in the Barents Sea D. robustus apparently never goes down to the shallowest depth (43 m) reported here for D. velifer. Thus, despite the possibility that the lower bathymetric limit of D. robustus might hypothetically overlap with the upper bathymetric limit of D. velifer at a general scale, these two species are unlikely to occur syntopically. Also, for the correct evaluation of the taxonomic placement of the Gullmar Fjord Dendronotus specimens, it is crucial that despite specimens of D. velifer (having general preference for deeper waters) in some localities are able to appear much shallower (for example following by low temperatures in shallow waters in many Arctic regions, e.g. in the Kara and White seas), D. robustus has never been recorded at depths greater than 20 m. Thus, along with morphological arguments, the results of the statistical analysis of the bathymetric distribution (Figure 4c) strongly support the hypothesis that it was D. velifer that occurred in the Gullmar Fjord and the southern Norwegian localities.